Microphone



W. A. BRUNO Sept. 20, 1938.

IICROPHONE Filed Dec. 29, 193

6 2 Sheets-Sheet 1 by R INVENTOR ATTORNEY 2 Shuts-Sheet 2 |NVEN;TOR 5 711 720 BY ATTORNEY W. A. BRUNO MICROPHONE Filed Dec. 29,, 19%

g Patented Sept. 20, 1938 UNITED STATES PATENT OFFICE.

The present invention relates to. improvements in microphones, it being understood that this term is used in its broad sisniilcance and that the. invention may be equally well applied to loud speakers, vibration recorders, such as cardiographs and the like and phonograph pickups.

The invention is concerned with the condenser type of microphone in which the condenser couple consists generally of a rigid or stationary back Plate and a conducting membrane electrically insulated from said back plate, but mechanically vibrating with respect thereto in order to vary the capacity of the condenser which willv ail'ect the circuit in which the microphone is included. In other words, the' condenser will translate small variations in pressure into related variations in electrical capacity.

It is well known that in microphones of the condenser type, variations in sound pressure upon the diaphragm displace it in its relation to the back plate, the diaphragm being insulated from the back plate and the diaphragm being usmliy separated from the back plate by the interposedair; the relative movement of the diaphragm causing. changes in the capacity existing between the diaphragm and back plate. Such variations are, by means of well known electrical circuits and devices. converted into sound waves, similar tothe original;

Devices'of this type are, however, subject to several inherent defects, some of which are low electrical output, sensitivity indhumidity and rapid changes in temperature and barometric pressure. It is an object of the present invention to improve the construcflon so as to eliminate, or greatly reduce, these defects, chiefly by slitting the diaphragm to convert same into a series of ribbons having independent and local vibratory movement of high responsiveness to both mechanical and electrical forum.

Another object of the invention consists in the method of support or mounting for the ribbon diaphrasm,which supp rt has for itschief purpose to maintain the diaphragm in its Proper relative position to the back plate, yet supporting the diaphragm in an condition.

A further object of the invention is to produce a: condenser couple in whichthe back plate is slotted and either covered or enveloped with a dielectric coating having a roughened, uneven, or

. non-uniform external surface against which the ribbondiaphragm m abuts,whereby theribbonsoi'thedlaphragmwillbeheldaaainst agreat numberofhlshpointsofthedielectrie coating rather than against any surface ent invention.

in the claims appended hereto.

In the drawings, wherein like symbols refer to like or correspondingparts throughout the is several views,

1'igure 1 is a front elevation, with parts broken away, of an improved microphone and its support, as constructed in accordance with the pres- 2o 1"lgure2isaverticalcross-secflontakenon the line 1-! in Figure 1.

Hguretisaplanviewoftheribbondiais a similar-view of the back-plate gs thebackplate'alongthelineS-dinl'igure! and showing the dielectric insulating coating.

Figure 'l is a circuit diagram showing one method of connecting the improved microphone in an electronic amplifying circuit. -I'igure8isaplanviewofamodifiedformof back plate.

l 'iguresisafrontelevatimwlthpartsbmken away, showing a modified form of microphone intended for translation of mechanical vibra- 40 tion.

Figure loisavertical sectiontaken onthelineiO-ll in l'igure 9.

Figure 11 is asideelevatiomwlthpartshroken awayandpartsshowninsection,ofthehnproved 45 condenser shown as applied to a pph arm, and V Figure 12 is a horizontal section, taken on an enlarged or scale, through a portion of the phonograph arm and improved 5 along the line iI-i! in Figure 11.

Referring more particularly to the drawings, andiorthepresenttol'iguresltofiinclusive. these is shown one embodiment of the improved microphoneinvolvingacondemercouplecanf diaphragm I6.

plate for the free movement of the air resulting from the act of vibration in the metallic resilient As shown in Figure 6, the back plate I! is covered by a coating l8 of adielectric insulating material. This material will be molded or otherwise formed about the back plate without interierence to the slots or openings l1 therethrough; such material naturally forming a roughened, uneven or non-uniform surface l9 against which the diaphragm IE will mechanically bear, whereby such diaphragm will be supported by high points 20 of the dielectric, which high points are adjacent low points 2|, which will prevent the diaphragm from cleaving or clinging to the back plate or its coating.

Referring to Figures 3 and 4, it will be seen that along the upper portions thereof the back plate I! and the diaphragm i6 are provided with perforations 22 and 23 respectively, which register with one another whereby to receive appropriate fastening means for binding the two electrodes together.

Referring to Figure 3 the diaphragm i8 is slitted as indicated at 24, along a number 01! parallel longitudinally extending lines which may or may not be equidistant from one another, thereby producing a plurality of ribobns 25. The

.slits 24 extend over the. major portion of the diaphragm i6 and substantially convert this diaphragm from a plate into a series of independently vibrating ribbons 25. The independence of these ribbons is assured by the fact that the slits 24 extend completely through one edge of the diaphragm l6 so that the only connection the ribbons have with the diaphragm i8 is at their upper edges. This support is narrow, and due to the thinness of the diaphragm l6, such ribbons will be very sensitive to mechanical and electrical forces tending to set up vibrations therein. The local vibratory quality of the elongated ribobns 25 will be greatly increased. The independence of theribbons one from another and from the former construction of a solid plate, is further enhanced by the provision of cut-away portions 26 in the diaphragm It at the inner ends of the slits 24. These cut away portions 26 are for the purpose of allowing freedom of pivotal or hinging movement of the ribbons 25 with the blank head portion of the diaphragm it which serves as a suspending support for the various ribbons as well as an attaching means for the diaphragm to the plate and to the electrical circuit. The diaphragm is wholly unstretched and is supported only by the head-piece it so that the ribbons 25 have a new freedom of movement over anything heretofore known in the art.

Where slots, such as I1, are-produced in the back plate l5, such slots will be of such a width as to be spanned by the ribbons 25, and the slots l1 will preferably be arranged in vertical rows,

. each ribbon being sufliciently long to extend over all of the slots of a vertical row, and the major areas of the ribbons being exposed over the slots i1 while the edge portions of the ribbons 25 rest against the uneven surfaces of the dielectric coating l8 at opposite sides of such slots.

From a comparison of Figures 3 and 4 it will be noted that the diaphragm I6 is narrower than the back plate I5. This is for the purpose of enabling free edge portions of the plate i5 to be received in the side clamps which support the condenser in a casing 21, which is a usual form of microphone casing supported upon the hollow standard 28.

The clamps are indicatedat 29 and the same are held by the screws 30 or other fastenings to the side walls of the casing 21. In the clamps are channel-shaped insulating bushings 3| which are in immediate contact with the back plate I5. These clamps are shown to be four in number, two at each side of the case. These clamps support the back plate l5 directly from the casing 21. The diaphragm I6 is supported by the back plate I 5 in insulated relation therefrom but bear ing mechanically against the dielectric coating as heretofore described.

While the support may be of any suitable character, I prefer to employ a metallic bar 32 having openings therethrough in registry with the openings .22 and 23 of the back plate i5 and diaphragm 5, whereby screws or other fastenings 33 may be passed through the aligning openings in these three members and secured in place by means of nuts 34 or the like. Preferably an insulating strip 35 will be interposed between the head-piece of the diaphragm l6 and the upper portion of the back plate l5 to insure that the insulation does not break down at this point where the pressure of the fastening means is being tightened up is apt to squeeze through the dielectric coating. Also insulating sleeves 36 surround the screws 33, their heads and the nuts 34. The

metallic bar 32 is of course in direct electrical contact with the head-pieceof the diaphragm it against which it directly bears. ing post 31 may be carried directly on this metallic bar 32 in order to connect the microphone at the diaphragm side in the electrical circuit, and a lead 38 is shown as connected to this binding post 31. Another lead 39 is shown as connecting with a screw or binding post4|l mounted in direct metallic contact with the metal body of the back plate ii. The leads 38 and 29 pass down through a bushing 4|, which serves to mount the casing 21 upon the hollow standard 28, and through this hollow standard to the external circult.

The wire guards are shown at 42 the same being affixed to, or carried by, the frames 43, these frames being mounted upon the open ends of the case 21 as by use of the screws or other fastenings 44. These guards 42 will protect the interior parts in a well understood manner.

Referring more particularly to Figure 8, a modified form of back plate l5 is shown in which perforations il replace the slots ll of Figure 4. The upper portion of this plate is provided with perforations 22* similar to the perforations 22 of Figure 4 and have a like purpose. The ribbons 25 of the diaphragm are laid across the perforations ll of this modified form of back plate. or course insulating material, similar to the insulating material l8 of Figure 6, will surround the perforations l1- and create raised points between the perforations H, such raised points occurring as indicated at 45.

Referring more particularly to Figure 7, the improved microphone is designated generally by its case 21. The terminal 40 of back plate i5 is connected to the negative side of the direct cur- A screw or bindworking in-conjunction with filtering resistor l8,

tend to prevent voltage fluctuations from power 1 supply 46 affecting the polarizing potential, which it supplies to microphone '21, and alternating current fluctuations caused by movement of the diaphragm 18 going through power supply 46 and other parts of the circuit. A coupling condenser ill allows the voltage variations, produced by movement cliche-microphone diaphragm l6, appearin'g across resistor ll, to be impressed upon the grid ii of vacuum tube 52. A resistor 53 furnishes the return path for the grid circuit. A resistor 54 is used to furnish bias for tube 52 and the condenser 55 to smooth out any possible variations in this bias. The circuits following from the plate in the hollow arm or tube 52 are well known in the electronic art and need not be described.

Movement of the diaphagm it of the microphone causes charging and discharging current to flow through resistor 41 due to polarizing potentials supplied by the power supply 46 and the change in capacity of the condenser formed between the back plate i5 and the ribbon diaphragm l8. This voltage, appearing across the resistor I1, is then impressed upon the grid M of the tube 52, and amplified accordingly.

Referring more particularly to Figures 9 and 10, there is shown a form of device which is of use in translating mechanical vibrations into equivalent electrical variations.

The casing 56 is placed in mechanical contact with the vibrating object and these vibrations are in turn transmitted to the ribbon diaphragm and back'plate through the mounting studs 51. Thus the mechanical vibrations set up similar vibrations in the ribbons of the diaphragm and in the back plate assembly, causing corresponding variations incapacity between the two electrodes. These variations may be translated by means of a circuit, such for instance as that illustrated in Figure 7, into corresponding electrical vibrations.

In this case the condenser unit is the same as heretofore described in connection with Figures r l to 6 of the drawings and involves the back plate IS, the ribbon diaphragm it, the bar 32 and the insulating strip 35 together with the means for holding these parts together as a unit. In this instance, instead of the condenser unit being sup ported to the casing by means of side clamps, the unit is attached directly to the stud 51 as by the use of screws or other fastenings 58 passing through insulating sleeves 59. The condenser unit is connected to the external circuit by leads connecting with the two electrodes as shown in Figure 9, and aspreviously explained.

The front portion of the casing 56 is covered by a metallic screening 60 held in place by'a front frame 6| which is removably secured to the easing as by means of the screws or other fastenings 62. This screening forms a protection for the condenser unit and other interior parts of the device.

Referring more particularly to Figures 11 and 12, I here show a modification of the condenser unit for-use as a phonograph pickup. In this connected thereto. This screw iii passes through the two clamp bars or blocks 68 and 89. The clamp bar or block 89 will be threaded to receive the threads of the screw 85. The other clamp bar 68 may have a plain or unthreaded bore for the free passage of the bolt or screw 66 there through. 'An insulating grommet l surrounds the shank of the bolt 86 and extends up on the opposite faces of the back plate l between such plate and the adjacent end portions of the two diaphragms thus effectively insulating the diaphragins from the back plate. The clamp blocks or bars 68 and 69 are in electrical connection both with the two diaphragms and with the bolt 66.

Bearing against the device, and preferably against one of the clamp blocks or bars 89, resilient damping block 'li inserted at this po t to dampen out undesired oscillations in the mechanical system. The resilient damping block it engages at its outer side against a fixed part, for instance the swinging arm 12 of a phonograph mechanism.

As shown in Figure 11 the condenser assembly is pivoted within the hollow phonograph arm it on the pivots 13. This enables the entire assembly to move on a vertical axis such as is necessary for the needle 64 in following the variations in the groove of the record disc M. The record is carried upon a revolving turn-table l5 driven through a shaft 16 in a manner well known to the mechanical art. The phonograph arm 12 is in the usual manner pivotally carried by a support ll, whereby the needle may traverse the groove in the record H from the outer edge of the disc to a central portion as required by the volute form of the groove. I

As the needle 54 is moved from side to side from its static position in following the variations in the groove of the recorded disc, such variations are transmitted as vibrations to the assembly, including the back plate and the two ribbon diaphragms, causing proportional changes in capacity between them. These changes in capacity are again translated into similar electrical variations, for instance by the circuit arrangement shown in Figure 7.

The use of the device as a microphone is believed to be clear. The circuit arrangement, such as shown in Figure 7, impresses the'output of the microphone upon the grid of the amplifying tube. Variations in capacity between the ribbon diaphragm and the back plate, caused by acoustic inputs to the device, draw charging current through the series resistance 41 due to polarizing potential applied through this resistance. Such variations in voltage are impressed upon the grid iii of the tube 52 in a conventional manner.

While the components of the new device and the recognized condenser microphone, are similar, these components have been altered in a manner to produce a superior effect. The invention eliminates the necessity for the use of a massive and accurately-machine back plate. It also eliminates the necessity for a stretcheddiaphragm. The back plate 15 may consist of a relatively thin metallic plate. The movable element, namely, the ribbon diaphragm i6, may

consist of an extremely thin aluminium sheet, for 75 instance thinner than one-thousandth of an inch. This ribbon diaphragm is not restricted in its movement in that it is'held physically in position only at the point of contact with its external connection.

It is preferable to entirely insulate the back plate, and by means of a high dielectric constant material which surrounds the plate in its entirety with the exception of the point of external electrical contact to the plate, which in thiscase is at ground potential.

As previously stated the insulating material is raised at a variety of points to ofier support for the movable element and to prevent the diaphragm ribbons from adhering to the insulation of the back plate due to electrostatic attraction when a potential difference is applied between the electrodes. Those portions of the back plate diaphragm, which are separated by the high dielectric constant insulating material covering bill the back plate, and air constitute the working or changeable portion of the capacity thus formed between the conducting elements. The slots ill or perforations W of the back plate are so arranged that small and rapid variations'in atmospheric pressure, such as constitute sound waves, are tree to move the ribbbons of the diaphragm toward and away from the back plate.

As the capacity of a condenser increases inversely as the square of the distance between its plates, and as in the present improved device the average d stance between the active portions or the ribbons and the areas involved can be made so much larger than in any stretched diaphragm type of condenser microphone, the improved device can be made to have a much greater output than is possible through any other means. Thus, the resultant capacity changes will be relatively large due to the disposition of the ribbons, the large areas involved (by comparison to the stretched diaphragm type unit) and the ease with which the movable portions or the ribbons are free to move...

It be noted that in the improved device the diaphragm is unstretched and that it is held in position preferably at one point or along one edge. The diaphragm is slitted so as to operate on both the pressure and the velocity gradient. The back plate is perforated or slotted so that the device operates on the velocity gradient of the sound waves.

Whenever the term microphone is used in this description or in the following claims, it will understood to applicable not only to a send instrument but also to receiving instruments and to loud speakers, as well as to the other h1- strumentsillustrated and described herein.

it is obvious that various changes and modifi cations may "be made in the details of construc= tion and design of the above specifically de= scribed embodiment of this invention without departing from the spirit thereof, such changes and modifications beingrestricted only by the scope or the following claims,

What is claimed is:--

i. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric of uneven surface, adiaphragm iree- 1y supported and contacting only the high points of said dielectric for preventing said diaphragm from clinging to said back plate, said diaphragm being slitted along a number of parallel 1ongi-= tudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at their upper edge, said upper edge being narrow and said diaphragm being relatively thin, said diaphragm being further cutaway at the inner ends of said slits for providing freedom of pivotal movement 01. said ribbons, and slots in said back plate arranged to completely be spanned by said ribbons, said diaphragm being narrower than said back plate, said diaphragm being secured to said plate solely at its upper end.

2. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric of uneven surface, a diaphragm freely supported and contacting only the high points of said dielectric for preventing said diaphragm from clinging to said back plate, said diaphragm being in the form of a series of ribbons, each ribhon being supported at one end and free for vibration at the other end, said upper edge being narrow and said diaphragm being relatively thin, said diaphragm being further cut away at the inner ends of said slits forprovldine freedom of pivotal movement of said ribbons, and slots in said back plate arranged to be completely spanned by said ribbons, said diaphragr n being narrower than said back plate, said diaphiagm being secured-to said plate solely at its upper end.

3. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric of uneven surface, a diaphragm freely supported and contacting only the high points of said dielectric ior preventing said diaphragm from clinging to said back plate, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at their upper edge, and slots in said back plate arranged to be completely spanned by sale. ribbons, said diaphragm being narrower than said baclr. plate, said diaphragm being secured to said plate solely at its upper end. v

l. In a translating device, a condenser couple,

a bacl: plate, said plate being coated with l a dielectric or uneven surface, a diaphragm freely supported and contacting only the high points of said dielectric for preventing said diaphragm from clinging to said back plate, said diaphragm being slitted along number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge or the diaphragm so that the onlyconnection the ribbons have with the diaphragm is at their upper edge, said diaphragm being secured to said plate solely at its upper end.

' 5. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric oi uneven surface, and a diaphragm freely supported and contacting only the high points of said dielectric for preventing said diaphragm from clinging to said back plate, said diaphragm being in the form of a series of ribbons, each ribbon being supported at one end and tree for vibration at the other end.

6. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric of uneven surface, a diaphragm freely supported and contacting only the high points of said dielectric for preventing said diaphragm from clinging to said back plate, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diap m is at their pper edge, slots in said back plate, said diaphragm being secured to said plate solely at its upper end.

7. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric, a wholly unstretched diaphragm freely supported, said diaphragm being slitted along a number of parallel longitudinally extending lines form-ing ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at their upper edge, said diaphragm being secured to said plate solely at its upper end.

8. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric, a wholly unstretched diaphragm freely supported, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragmis at their upper edge, said diaphragm being further cut away at the inner ends of said slits for providing freedom of pivotal movement of said ribbons, said-diaphragm being secured to said plate solely at its upper end.

9. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric, a wholly unstretched diaphragm freely supported, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at the upper edge, said diaphragm being further cut away at the inner ends of said slits for providing freedom of pivotal movement of said ribbons, and slots in said back plate, said diaphragm being secured to said plate solely at its upper end.v

10. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric molded about said back plate, a wholly unstretched diaphragm freely supported, saiddiaphragmbeingslittedalonganumberof parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at the upper edge, said diaphragm being further cut away at the inner ends of said slits for providing freedom of pivotal movement of said ribbons, and slots in said back plate, said'diaphragm being secured to said plate solely at its upper end.

11. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric, a diaphragm freely supported, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending throughone edge of the diaphragm so that the only connection the ribbons have with the diaphragm is at their upper edge, and slots in said back plate arranged to be completely spanned by said ribbons, said diaphragm being narrower than said back plate, said diaphragm being secured to said plate solely at its upper end.

12. In a translating device, a condenser couple, having a back plate, said plate being coated with a dielectric, a diaphragm freely supported, said diaphragm being slitted along a number of parallel longitudinally extending lines forming ribbons, said slits extending through one edge of the diaphragm'so that the only connection the ribbons .have with the diaphragm is at their upper edge, and slots in said back plate, said diaphragm being secured to said plate solely at its upper end. I

said slits extending through one edge of the dia- I phragm so that the only connection the ribbons have with the diaphragm is at their upper edge,

said diaphragm being secured to said plate solely at its upper and.

-- Will-HAM A. BRUNO. 

